It is hitherto known that an oligomerization of .alpha.-olefin such as ethylene is carried out in the presence of a chromium-based catalyst comprising a specific chromium compound and a specific organoaluminum compound in combination. For instance, in Japanese Patent Publication (KOKOKU) No. 43-18707, there is described a process for producing 1-hexene and polyethylene from ethylene by using a chromium-based catalyst comprising a compound of VIB-Group transition metal such as chromium and polyhydrocarbyl-aluminum-oxide.
Further, in Japanese Patent Application Laid-Open (KOKAI) No. 3-128904, there is described a method for the trimerization of an .alpha.-olefin by using a chromium-based catalyst obtained by reacting a chromium-containing compound having chromium-pyrrolyl bond with an alkyl metal or a Lewis acid. Furthermore, in South Africa Patent No. ZA93/0350, there is described a method for the trimerization of ethylene by using a chromium-based catalyst obtained by mixing a chromium compound, a pyrrole-containing compound, an alkyl metal and a halide source with each other in a common solvent.
On the other hand, the present inventors have previously proposed, in Japanese Patent Application Laid-Open (KOKAI) No. 6-145241, a method for the oligomerization of an .alpha.-olefin by using a chromium-based catalyst comprising a chromium-containing compound having chromium-pyrrole bond and alkyl aluminum in combination in such a manner that the chromium-containing compound and the alkyl metal compound are kept in non-contact with each other until these compounds are brought into contact with an .alpha.-olefin. According to the afore-mentioned method, 1-hexene can be produced by subjecting the .alpha.-olefin, particularly ethylene, to oligomerization in the presence of the catalyst having a high catalytic activity.
Further, the present inventors have also proposed, in Japanese Patent Application Laid-Open (KOKAI) No. 6-157655, a method for the oligomerization of an .alpha.-olefin in the presence of a chromium-containing compound produced by mixing a chromium compound with a pyrrole ring-containing compound in a hydrocarbon solvent, in which the contact between the chromium-containing compound and the alkyl aluminum compound is conducted in the same manner as described above. According to this method, 1-hexene having a high purity can be produced by subjecting an .alpha.-olefin, particularly ethylene, to trimerization in the presence of the catalyst having a high catalytic activity.
In addition, the present inventors have recently proposed, in Japanese Patent Application Laid-Open (KOKAI) No. 8-3216, a method of the oligomerization of an .alpha.-olefin by using a chromium catalyst comprising a chromium-containing compound, a pyrrole ring-containing compound, a alkyl metal compound and a halide source in combination, in such a manner that the chromium-containing compound and the alkyl metal compound are kept in non-contact with each other until these compounds are brought into contact with an .alpha.-olefin. According to this method, 1-hexene can be produced by subjecting the .alpha.-olefin, particularly ethylene, to oligomerization in the presence of the catalyst having a high catalytic activity.
However, in the method described in Japanese Patent Publication (KOKOKU) No. 43-18707, a large amount of polyethylene is produced simultaneously with the production of 1hexene. Whereas, in the case where the reaction condition is adjusted so as to reduce the amount of polyethylene as a by-product, there is caused such a problem that the catalytic activity of the catalyst is deteriorated. In the method described in Japanese Patent Application Laid-Open (KOKAI) No. 3-128904, there arises a problem in which although the amount of high-molecular weight polymer produced can be suppressed to a small level, the catalytic activity of the catalyst used is unsatisfactorily low.
In addition, in the method described in the South Africa Patent No. ZA93/0350, the catalytic activity of the catalyst used therein is still insufficient from a standpoint of conducting the oligomerization of .alpha.-olefin in an industrial-scale, though the method exhibits a high selectivity for 1-hexene. Further, in the method described in the Japanese Patent Application Laid-Open (KOKAI) Nos. 6-145241 and 6-157655, the catalytic activity of the catalyst used therein is also still insufficient from a standpoint of conducting the oligomerization of .alpha.-olefin in an industrial-scale. On the other hand, in the method described in the Japanese Patent Application Laid-Open (KOKAI) No. 8-3216, the selectivity for 1-hexene is insufficient, though the catalyst used in the method exhibits a sufficient catalytic activity to conduct the industrial-scale oligomerization of .alpha.-olefin.
The present invention aims at solving the afore-mentioned problems. An object of the present invention is to provide a process for producing an .alpha.-olefin oligomer such as 1-hexene with an extremely high yield and a high selectivity in an industrially advantageous manner.